It is often desirable to determine values of unknown DC voltage levels or time-varying signals without using an oscilloscope or an external voltmeter. It is also often desirable to make use of such values in a number of ways, such as determining a stable triggering point for repetitive signals, or making peak-to-peak voltage measurements, or making risetime and falltime measurements of pulse edges, et cetera. Such situations arise in the use of electronic counter or timer apparatus, wherein applied unknown signals or voltage levels are measured to determine such characteristics as frequency, elapsed time between electrical events, numbers of electrical events occurring, and so forth. Such electronic counter apparatus typically have an input voltage window to limit the positive and negative peak amplitudes to a predetermined dynamic range, and a triggering level within such triggering window through which a signal must pass to generate a count response by the counter circuits. An operator manually adjusts the triggering level by means of a front-panel control to set the triggering level at a point which will result in stable triggering. If the overall signal amplitude is much smaller than the triggering window, some hunting with the front-panel control is necessary to locate the signal, followed by trial-and-error adjustments to stabilize the count.
This problem has been recognized by other investigators in the art, and automatic level-detecting and triggering apparatus and methods have been developed. One such automatic triggering circuit is described in U.S. Pat. No. 4,121,164, which is assigned to the assignee of the present invention. Here, the trigger level voltage is automatically swept through the triggering window until the input signal voltage level is encountered. However, the sweeping of the triggering level continues so that triggering occurs at different arbitrary levels if the input signal is time varying.
U.S. Pat. No. 4,069,452 teaches an apparatus which automatically detects the precise values of upper and lower peaks, as well as the mid-point, of a time-varying signal. A series of stairstep voltage ramps scan predesignated ranges within the triggering window to encounter the input signal. When the general area of positive and negative peaks are determined, a smaller stair voltage ramp is used to precisely (within the step increment) locate the peaks. The midpoint between peaks may then be computed. This is a rather lengthy coarse-fine process, particularly if the input signal is changing very slowly or is a low-frequency repetitive signal. Furthermore, this system does not lend itself to accurate detection of fast electrical spikes occurring at a very low repetition rate, it cannot detect out-of-range signals, nor can it determine DC voltage levels because there is no facility for changing slope to allow sweeping in on the voltage level from both directions. In other words, a voltage level may be detected, but it is unknown whether such level is actually a signal peak or a DC voltage.